Method for producing methacrylic acid from isobutane

ABSTRACT

A process prepares methacrylic acid from isobutane by subjecting isobutane to a partial catalytic dehydrogenation in the gas phase and charging an oxidation zone with the isobutenic product gas mixture after the components other than isobutane and isobutene have been substantially removed from the product gas mixture. The oxygen required to charge the oxidation zone is introduced accompanied by nitrogen.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing methacrylicacid from isobutane by

-   A) subjecting the isobutane in a reaction zone A to a partial    selective heterogeneously catalyzed dehydrogenation in the gas phase    to form a product mixture A which comprises isobutene and    unconverted isobutane,-   B) using the isobutane- and isobutene-containing product gas mixture    A to charge a reaction zone B and subjecting the isobutene in    reaction zone B to a selective heterogeneously catalyzed partial    oxidation in the gas phase using molecular oxygen to form a    methacrolein-containing product gas mixture B with the proviso that    the molar conversion of isobutene is ≧95 mol % and-   C) using the methacrolein-containing product gas mixture B without    preceding removal of components contained therein to charge a    reaction zone C and subjecting the methacrolein in reaction zone C    to a selective heterogeneously catalyzed partial oxidation using    molecular oxygen in the gas phase to form a methacrylic    acid-containing product gas mixture C.

2. Discussion of the Background

Methacrylic acid is an important staple chemical which is used as suchand/or in the form of its methyl ester for preparing polymers which areused, for example, finely dispersed in an aqueous medium as a binder.

DE-A 3313573 discloses a process for preparing methacrylic acid fromisobutane as described at the outset. A procedure consideredparticularly advantageous by DE-A 3313573 involves using the product gasmixture A without preceding removal of components contained therein forcharging reaction zone B and using pure oxygen as the source for themolecular oxygen required in reaction zone B.

However, disadvantages of such a procedure are that on the one hand theselectivity of methacrylic acid formation per se and on the other handthe extent of formation of the by-produced isobutyraldehyde andisobutyric acid, which are particularly undesirable even in very smallquantities since they are difficult to remove from the target productand are particularly troublesome in subsequent uses, is not completelysatisfactory.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forpreparing methacrylic acid from isobutane as described at the outsetwhich only has the disadvantages described to a reduced extent.

DETAILED DESCRIPTION OF THE INVENTION

We have found that this object is achieved by a process for preparingmethacrylic acid from isobutane by

-   A) subjecting the isobutane in a reaction zone A to a partial    selective heterogeneously catalyzed dehydrogenation in the gas phase    to form a product mixture A which comprises isobutene and    unconverted isobutane,-   B) using the isobutane- and isobutene-containing product gas mixture    A to charge a reaction zone B and subjecting the isobutene in    reaction zone B to a selective heterogeneously catalyzed partial    oxidation using molecular oxygen in the gas phase to form a    methacrolein-containing product gas mixture B with the proviso that    the molar conversion of isobutene is ≧95 mol % and-   C) using the methacrolein-containing product gas mixture B without    preceding removal of components contained therein to charge a    reaction zone C and subjecting the methacrolein in reaction zone C    to a selective heterogeneously catalyzed partial oxidation using    molecular oxygen in the gas phase to form a methacrylic    acid-containing product gas mixture C,    which comprises    removing at least 80 mol % of the components other than isobutane    and isobutene from the isobutane- and isobutene-containing product    gas mixture A before it is used to charge reaction zone B, and    introducing the molecular oxygen required in reaction zone B to    reaction zone B accompanied by molecular nitrogen in a molar ratio R    of molecular oxygen to molecular nitrogen of from 1:1 to 1:10.

In other words, according to the invention, R may be 1:2, or 1:3, or1:4, or 1:5, or 1:6, or 1:7, or 1:8, or 1:9, or 1:10. An advantageousratio R is in the range from 1:2 to 1:5 or in the range from 1:3 to 1:5,or from 1:3.5 to 1:4.5. In the process according to the invention, theabovementioned accompaniment of the molecular oxygen by molecularnitrogen is advantageously realized in such a manner that the molecularoxygen is introduced into reaction zone B as a component of a gas whichalready comprises the molecular oxygen and molecular nitrogen in anabovementioned ratio R or consists only of molecular oxygen andmolecular nitrogen in such a ratio R. In the process according to theinvention, preference is given to at least partially, more preferablypredominantly or exclusively, using air as the source for the molecularoxygen required in reaction zone B. However, it is also possible tointroduce to reaction zone B, for example, air and additionallymolecular oxygen or air and additional molecular nitrogen or air andadditionally a mixture of molecular nitrogen and molecular oxygen whichcomprises the two gas components in a ratio other than that in air. Itis essential to the invention only that the ratio R be maintainedoverall. According to the invention, molecular oxygen and molecularnitrogen could also be introduced spatially separated to the reactionzone B.

In other words, while DE-A 3313573 teaches the selective heterogeneouslycatalyzed partial oxidation of isobutene in reaction zone B in areaction gas mixture which comprises in particular iso-butene, isobutaneand molecular oxygen, the reaction gas mixture in reaction zone B in theprocess according to the invention necessarily contains isobutene,isobutane, molecular oxygen and molecular nitrogen. The additionalpresence of the latter in the process according to the inventionsurprisingly ensures a reduction in the undesired conversion ofisobutane to undesired by-product.

A further feature essential to the invention consists in the removal ofat least 80 mol % of the components other than isobutane and isobutenefrom the isobutane- and isobutene-containing product gas mixture Abefore it is used to charge reaction zone B. The molar reference basisis the sum of the molar amounts of the different individual componentscontained in the product gas mixture B. According to the invention, theabovementioned removal does not have to be homogeneous over thedifferent components. Rather, it may capture individual componentsquantitatively and others only partially. Only the sum total has toachieve the abovementioned percentage of 80 mol %. According to theinvention, preference is given to removing at least 85 mol %, morepreferably at least 90 mol %, even more preferably at least 95 mol %,even better 97 mol % and at best at least 99 mol % or 100 mol %, of thecomponents other than isobutane and isobutene from the product gasmixture A before it is used to charge reaction zone B.

It will be appreciated that the charging gas mixture (=the mixture ofall gas streams introduced into the reaction zone) of reaction zone B inthe procedure according to the invention comprises, in addition to thecomponents already mentioned, other components, for example, CO, CO₂,H₂O, noble gases such as He and/or Ar, hydrogen, methane, ethylene,ethane, butanes, butenes, butynes, pentanes, propyne, allenes, propane,propylene, acrolein and/or methacrolein.

According to the invention, the molecular nitrogen content of thecharging gas mixture of reaction zone B, based on the amount ofisobutene contained in this charging gas mixture, should not be lessthan 500 mol %. In other words, the molecular nitrogen content of thecharging gas mixture of reaction zone B in the process according to theinvention, based on the amount of isobutene present, can be at least 500mol %, or at least 600 mol %, or at least 700 mol %. However, the ratioof the molar quantity of molecular nitrogen contained in the charginggas mixture of reaction zone B to the amount of isobutene contained inthe charging gas mixture of reaction zone B according to the inventionwill normally be ≦20:1, frequently ≦12:1.

The molar ratio of the amount of molecular nitrogen contained in thecharging gas mixture of reaction zone B to the amount of isobutane inthe charging gas mixture of reaction zone B in the process according tothe invention will generally not be less than 1:1. Normally, this ratiowill also not be above 16:1.

In other words, the molar ratio of the amount of molecular nitrogencontained in the charging gas mixture of reaction zone B to the amountof isobutane contained in the charging mixture for reaction zone B can,according to the invention, be from 1:1 to 16:1, or from 2:1 to 10:1, orfrom 2:1 to 4:1.

Frequently, the composition of the charging gas mixture of reaction zoneB in the process according to the invention will be selected in such amanner that the following molar ratios are fulfilled:iso-butane:iso-butene:N₂:O₂:H₂O:H₂:others=10-40:4-8:20-70:5-20:0-20:0-5:0-5.

According to the invention, the abovementioned molar ratiosadvantageously=15-25:4-8:40-60:10-15:5-15:0-1:0.1-3. Frequently, theywill be 20:6:50:12:10:0:2.

An essential feature of the procedure according to the invention isthat, in contrast to the case of a homogeneously and/or heterogeneouslycatalyzed partial oxydehydrogenation of isobutane, molecular hydrogen isformed at least intermediately in reaction zone A, which is why theproduct gas mixture A generally comprises molecular hydrogen.Furthermore, the catalytic dehydrogenation in reaction zone A isendothermic without additional measures, while a catalyticoxydehydrogenation is exothermic.

According to the invention, before the product gas mixture A is used tocharge reaction zone B, at least 80 mol %, preferably at least 85 mol %,more preferably at least 90 mol %, even more preferably at least 95 mol%, or at least 97 mol %, or at least 99 mol %, and frequently the entireamount, of the molecular hydrogen contained therein will be removed.

In general, the molar ratio of isobutene contained in the product gasmixture A to the molecular hydrogen contained in the product gas mixtureA in the process according to the invention will be ≦10, customarily ≦5,frequently ≦3 and often ≦2.

Normally, the reciprocal of the abovementioned ratio will not exceed 2.In other words, the molar ratio of isobutene contained in the productgas mixture A to molecular hydrogen contained in the product gas mixtureA in the process according to the invention will customarily be ≧0.5,usually ≧0.8, and in many cases ≧1.2 or 1.5.

In contrast, the molar ratio of molecular hydrogen contained in thecharging gas mixture of reaction zone B (i.e. in charging gas mixture B)to the isobutene contained in the charging gas mixture B in the processaccording to the invention will generally be ≦1:10, customarily ≦1:50,often ≦1:100.

In addition to molecular hydrogen, components other than isobutane andisobutene contained in product gas mixture A are gases from the groupconsisting of N₂, CO, CO₂, H₂O, methane, ethane, ethylene, propane,propylene and also possibly O₂ and others.

It will be appreciated that in the process according to the invention,the abovementioned components will be removed to a substantial extentfrom the isobutane and isobutene contained in the product gas mixture Abefore it is used to charge reaction zone B (for example, at least 80mol %, or at least 85 mol %, or at least 90 mol %, or at least 95 mol %,or at least 97 mol %, or at least 99 mol %, thereof) or completely fromthe isobutane and isobutene contained in the product gas mixture A,generally in combination, i.e. normally together with the molecularhydrogen contained in the product gas mixture A. However, such combinedremoval is not vital according to the invention. Rather, more of onecomponent contained in the product gas mixture A and less of anothercomponent contained in the product gas mixture A may be removedaccording to the invention. However, according to the invention, atleast a portion of all of the components other than isobutane andisobutene will preferably be removed from product gas mixture A beforeit is used to charge reaction zone B.

In order to achieve interesting conversions in reaction zone A in thepartial heterogeneously catalyzed dehydrogenation to be carried outaccording to the invention, based on a single pass, operation generallyhas to be effected at relatively high reaction temperatures (typicallythese reaction temperatures are from 300 to 70° C.). Since thedehydrogenation (cleavage of C—H) is kinetically disadvantaged comparedto cracking (cleavage of C—C), it is effected on selective catalysts.For every isobutene molecule formed, a hydrogen molecule is generallyby-produced. As a consequence of the selective catalysts which arecustomarily configured in such a manner that they display significantdehydrogenation (at isobutane gas hourly space velocities of, forexample, 1000 h⁻¹ (1 at STP/1 of cat. h), the isobutene yield isgenerally at least 30% in a single pass (based on isobutane used)) withthe exclusion of oxygen at the abovementioned temperatures (for exampleat 600° C.), and by-products such as methane, ethylene, propane, propeneand ethane are only formed in insignificant amounts.

Since the dehydrogenation reaction proceeds with decreasing volume, theconversion may be increased by reducing the partial pressure of theproducts. This can be achieved in a simple manner, for example, bydehydrogenating at reduced pressure and/or by admixing in substantiallyinert diluent gases, for example steam, which is normally an inert gasfor the dehydrogenation reaction. Dilution with steam generally has thefurther advantage of reduced coking of the catalyst used, since thesteam reacts with coke formed by the principle of coal gasification.Also, steam may be used as a diluent gas in the subsequent oxidationstage B. Steam can also be readily removed partially or completely fromthe product gas mixture A of the process A according to the invention(for example by condensation) which opens up the possibility ofincreasing the proportion in reaction zone B of the diluent gas N₂,whose use is essential to the invention, in the further use of theproduct gas mixture A′ obtainable in this way. According to theinvention, it is entirely possible to use the entirety or else only aportion of the molecular nitrogen to be used in reaction zone Baccording to the invention, also for dilution in reaction zone A.Examples of further diluents for reaction zone A include CO, CO₂ andnoble gases such as He, Ne and Ar. However, operation in reaction zone Amay in principle also be effected without diluents; i.e. the charginggas mixture of reaction zone A may consist only of isobutane or only ofisobutane and molecular oxygen. All of the diluents mentioned may beused in reaction zone A either alone or in the form of highly varyingmixtures. According to the invention, it is advantageous that thediluents suitable for reaction zone A are generally also diluentssuitable for reaction zone B, so that their quantitative removal fromthe product gas mixture A is not indispensable to the invention. Ingeneral, preference is given to diluents which behave inertly (i.e. lessthan 5 mol %, preferably less than 3 mol % and even better less than 1mol % are chemically altered) in each reaction zone. In principle, alldehydrogenation catalysts known from the prior art are suitable forstage A according to the invention. They can be roughly divided into twogroups, i.e. those of oxidic nature (for example chromium oxide and/oraluminum oxide) and those which consist of at least one generallycomparatively precious metal (for example platinum) deposited on atleast one generally oxidic support.

Dehydrogenation catalysts which may be used for stage A according to theinvention are, inter alia, all of those recommended by DE-A 10060099(the example), WO 99/46039, U.S. Pat. No. 4,788,371, EP-A 705 136, WO99/29420, U.S. Pat. No. 5,220,091, U.S. Pat. No. 5,430,220, U.S. Pat.No. 5,877,369, DE-A 10047642, EP-A 117 146, DE-A 19 937 106, DE-A 19 937105, DE-A 10 211 275 and also DE-A 19 937 107. In particular, all thedehydrogenation process variants mentioned in this document as beingsuitable for reaction zone A according to the invention may be carriedout using the catalyst according to Example 1, and also according toExample 2, and also according to Example 3, and also according toExample 4 of DE-A 19 937 107.

These are dehydrogenation catalysts which comprise from 10 to 99.9% byweight of zirconium dioxide, from 0 to 60% by weight of aluminum oxide,silicon dioxide and/or titanium dioxide and from 0.1 to 10% by weight ofat least one element of the first or second main group, of an element ofthe third transition group, of an element of the eighth transition groupof the periodic table, of lanthanum and/or of tin, with the proviso thatthe sum total of the percentages by weight is 100% by weight.

The at least one catalyst bed (for example fluidized bed, moving bed orfixed bed) required for the purposes of the present invention maycontain the dehydrogenation catalyst in differing geometries. Examplesof useful geometries for the process according to the invention includeshapes such as spall, tablets, monoliths, spheres or extrudates (rods,wagonwheels, stars, rings).

In the case of extrudates, the length is advantageously from 2 to 15 mm,frequently from 2 to 10 mm, in many cases from 6 to 10 mm, and thediameter of the extrudate cross section is advantageously from 1 to 5mm, frequently from 1 to 3 mm. In the case of rings, the wall thicknessis advantageously from 0.3 to 2.5 mm, and the length is from 2 to 15 mm,frequently from 5 to 15 mm, and the diameter of the cross section from 3to 10 mm. A suitable shaping process is disclosed, for example, by DE-A10047642 and also DE-A 19937107. The process is based on the fact thatoxidic support materials admixed with concentrated mineral acid (forexample concentrated nitric acid) can be comparatively efficientlykneaded and can be converted by means of an extruder or an extrudatepress to an appropriate shaped body.

The shaped bodies are then dried and calcined and then salt solutionsare poured over them in a suitable sequence. Finally, they are againdried and calcined.

The reaction zone A relevant for the process according to the inventionmay in principle be realized in all reactor types known from the priorart for heterogeneously catalyzed partial dehydrogenations ofhydrocarbons in the gas phase over fixed-bed catalysts.

Typical reaction temperatures are from 200 to 800° C., or from 400 to650° C. The working pressure is typically in the range from 0.5 to 10bar. Typical gas hourly space velocities of reaction gas are from 300 to16 000 h⁻¹.

In principle, all reactor types and process variants known from theprior art may be used to embody reaction zone A of the process accordingto the invention. Descriptions of such process variants are containedin, for example, all prior art documents mentioned in relation to thedehydrogenation catalysts.

A comparatively comprehensive description of dehydrogenation processessuitable according to the invention is also contained in “Catalytica®Studies Division, Oxidative Dehydrogenation and AlternativeDehydrogenation Processes, Study Number 4192 OD, 1993, 430 FergusonDrive, Mountain View, Calif., 94043-5272 U.S.A.”.

A characteristic feature of the partial heterogeneously catalyzeddehydrogenation of isobutane is that it is endothermic. In other words,the heat (energy) necessary to achieve the required reaction temperaturehas to be supplied either in advance to the reaction gas and/or in thecourse of the catalytic dehydrogenation.

Also, owing to the high reaction temperatures required, it is typical ofheterogeneously catalyzed dehydrogenations of isobutane that smallamounts of high-boiling, high molecular weight organic compounds, up toand including carbon, are formed which deposit on the catalyst surfaceand thus deactivate it. In order to minimize this disadvantageous sideeffect, it is possible, as already mentioned, to dilute with steam theisobutane to be passed over the catalyst surface at elevated temperaturefor catalytic dehydrogenation. Under the resulting conditions,depositing carbon is partially or completely eliminated by the principleof coal gasification.

Another possible way of removing deposited carbon compounds consists inpassing an oxygen-containing gas through the dehydrogenation catalyst atelevated temperature from time to time and effectively burning off thedeposited carbon. However, it is also possible to suppress carbondeposit formation by adding molecular hydrogen to the isobutane to becatalytically dehydrogenated before it is passed over thedehydrogenation catalyst at elevated temperature.

It will be appreciated that the possibility also exists of adding amixture of steam and molecular hydrogen to the isobutane to becatalytically dehydrogenated. Addition of molecular hydrogen to thecatalytic dehydrogenation of isobutane also reduces the undesiredformation of by-produced allene, acetylene and other carbon precursors.

In the majority of processes known for heterogeneously catalyzed partialdehydrogenation of hydrocarbons such as isobutane to be dehydrogenated,the heat of dehydrogenation is generated outside the reactor andsupplied to the reaction gas from outside. However, this requirescomplicated reactor and process concepts and leads, particularly at highconversions, to steep temperature gradients in the reactor with thegeneral disadvantage of increased by-product formation.

Alternatively, the heat of dehydrogenation may also be generateddirectly in the reaction gas itself by adding molecular oxygen andexothermically combusting hydrogen formed either in the dehydrogenationor supplied additionally to give steam. To this end, a molecularoxygen-containing gas and optionally hydrogen are added to the reactiongas before and/or after entrance into the reaction zone containing thedehydrogenation catalyst. Either the dehydrogenation catalyst itself(this applies to most dehydrogenation catalysts) and/or any additionallyinstalled oxidation catalysts generally ease the desired hydrogenoxidation (cf. DE-A 10028582). In favorable cases, heat of reactionreleased in this manner by means of hydrogen combustion allows indirectreactor heating to be completely dispensed with and accordinglycomparatively simple process concepts and also limited temperaturegradients in the reactor even at high conversions.

In the above procedure, the use of external molecular hydrogen may, forexample, be avoided when the process principle of DE-A 10 211 275 isapplied.

According to this process principle, a reaction gas containing the atleast one hydrocarbon to be dehydrogenated (isobutane in this case) iscontinuously introduced into the catalytic dehydrogenation zone(reaction zone A in this case). In the catalytic dehydrogenation zone,the reaction gas is passed over at least one fixed catalyst bed wheremolecular hydrogen and some of at least one dehydrogenated hydrocarbon(isobutene in this case) are formed by catalytic dehydrogenation. Beforeand/or after entry into the catalytic dehydrogenation zone, at least onemolecular oxygen-containing gas which at least partially oxidizes themolecular hydrogen contained in the reaction gas in the catalyticdehydrogenation zone to give steam is added to the reaction gas. Aproduct gas mixture is then withdrawn from the catalytic dehydrogenationzone which comprises molecular hydrogen, steam, the at least onedehydrogenated hydrocarbon and the at least one hydrocarbon to bedehydrogenated, divided into two portions of identical composition andone of the two portions is returned to the catalytic dehydrogenationzone (cycle gas) as the source for molecular hydrogen.

In the process according to the invention, the other portion would bewithdrawn as product gas mixture A and, according to the removal of theinvention, introduced into reaction zone B.

It will be appreciated that reaction zone A in the process according tothe invention may also be configured in such a manner that there is afurther fixed catalyst bed downstream of the dehydrogenation catalystfixed bed in the flow direction of the reaction gas where molecularhydrogen contained in the reaction gas is at least partially combustedto steam by selective heterogeneous catalysis so that the product gasmixture A in the process according to the invention may be substantiallyor completely free of hydrogen. Catalysts suitable for this purpose aredisclosed by, for example, U.S. Pat. No. 4,788,371, U.S. Pat. No.4,886,928, U.S. Pat. No. 5,430,209, U.S. Pat. No. 55,530,171, U.S. Pat.No. 5,527,979, EP-A 832056 and U.S. Pat. No. 5,563,314.

A useful reactor form for reaction zone A according to the invention isthe fixed bed tubular or tube bundle reactor. In other words, thedehydrogenation catalyst and any specific hydrogen oxidation catalyst,as disclosed, for example, in the documents U.S. Pat. Nos. 4,788,372,4,886,928, 5,430,209, 5,550,171, 5,527,979, 5,563,314 and EP-A 832 056is disposed in a reaction tube or in a bundle of reaction tubes as afixed bed. The reaction tubes are customarily indirectly heated bycombusting a gas, for example a hydrocarbon such as methane, in thespace surrounding the reaction tubes. It is advantageous to apply thisindirect form of heating only to the first 20 to 30% of the fixed bedand to heat the remaining bed length to the required reactiontemperature using the radiative heat released in the combustion.Indirect heating of the reaction gas may be combined advantageously withdirect heating by combustion with molecular oxygen in the reaction gas.In this manner, a virtually isothermal reaction is achievable in acomparatively simple form.

Suitable reaction tube internal diameters are from about 10 to 15 cm. Atypical dehydrogenation tube bundle reactor comprises from 300 to 1000reaction tubes. The temperature in the reaction tube interiors is in therange from 300 to 700° C., preferably in the range from 400 to 700° C.The working pressure is customarily in the range from 0.5 to 8 bar,frequently from 1 to 2 bar or else from 3 to 8 bar. Advantageously, thereaction gas is introduced into the tubular reactor preheated to thereaction temperature. In general, the product gas mixture leaves thereaction tube at a (higher or lower) temperature other than the entrancetemperature (cf. also U.S. Pat. Nos. 4,902,849,4,996,387 and 5,389,342).For the purposes of the abovementioned procedure, it is advantageous touse oxidic dehydrogenation catalysts based on chromium oxide and/oraluminum oxide. Frequently, no diluent gas will be used, and insteadsubstantially pure isobutane will be used as the starting reaction gas.The dehydrogenation catalyst is usually also used undiluted. Typicalisobutane gas hourly space velocities are from 500 to 2000 h⁻¹ (=1 atSTP/l of catalyst h).

On the industrial scale, about three tube bundle reactors would beoperated in parallel, two of which would generally be carrying outdehydrogenation, while one of the reactors regenerates the catalystcharge.

It will be appreciated that reaction zone A according to the inventioncan also be configured within a moving bed. For example, the movingcatalyst bed may be accommodated in a radial flow reactor. In this, thecatalyst moves gradually from top to bottom while the reaction gasmixture flows radially. This procedure is used, for example, in the UOPOleflex dehydrogenation process. Since the reactors in this process areoperated virtually adiabatically, it is advantageous to operate morethan one reactor in series (typically up to four). This allowsexcessively high differences in the temperatures of the reaction gasmixture at the reactor entrance and reactor exit to be avoided (in theadiabatic mode, the starting reaction gas mixture functions as a heatcarrier on whose heat content the reaction temperature is dependent)and, despite this, attractive overall conversions to be achieved.

When the catalyst bed has left the moving bed reactor, it is regeneratedand then reused. An example of a dehydrogenation catalyst for thisprocess is a spherical dehydrogenation catalyst which consistssubstantially of platinum on a spherical aluminum oxide support. Inorder to avoid premature catalyst aging, hydrogen is advantageouslyadded to the isobutane to be dehydrogenated. The working pressure istypically from 1 to 5 bar. The hydrogen to isobutane (molar) ratio isadvantageously from 0.1 to 1. The reaction temperatures are preferablyfrom 550 to 650° C. and the gas hourly space velocity of reaction gasmixture is from about 200 to 1000 h⁻¹. The catalyst charge may alsoconsist of a mixture of dehydrogenation and H₂ oxidation catalysts, asrecommended by EP-A 832 056.

In the fixed bed processes described, the catalyst geometry may likewisebe spherical, or else cylindrical (hollow or solid).

A further process variant for reaction zone A according to the inventionis described by Proceedings De Witt, Petrochem. Review, Houston, Tex.1992 a, N1, which contemplates the possibility of a heterogeneouslycatalyzed dehydrogenation in a fluidized bed without diluting theisobutane.

This variant advantageously involves operating two fluidized beds inparallel, of which one is generally in the process of regeneration. Theactive composition used is chromium oxide on aluminum oxide. The workingpressure is typically from 1 to 1.5 bar and the dehydrogenationtemperature is generally from 550 to 600° C. The heat required for thedehydrogenation is introduced into the reaction system by preheating thedehydrogenation catalyst to the reaction temperature. The workingpressure is regularly from 1 to 2 bar and the reaction temperaturetypically from 550 to 600° C. The above dehydrogenation method is alsodisclosed in the literature as the Snamprogetti-Yarsintez process.

As an alternative to the above-described procedures, reaction zone Aaccording to the invention may also be realized according to a processdeveloped by ABB Lummus Crest (cf. Proceedings De Witt, Petrochem.Review, Houston, Tex., 1992, P1).

Common to the heterogeneously catalyzed dehydrogenation processes forisobutane described hitherto is that they are operated at isobutaneconversions of >30 mol % (in general ≦70 mol %) (based on single reactorpass).

An advantage of the present invention is that it is sufficient for theprocess according to the invention for an isobutane conversion of from≧5 mol % to ≦30 mol % or ≦25 mol % to be achieved in reaction zone A. Inother words, reaction zone A may also be operated at isobutaneconversions of from 10 to 30 mol % according to the invention (theconversions relate to single reactor pass). Among other factors, thisresults from the dilution with molecular nitrogen of the remainingamount of unconverted isobutane in the downstream reaction zone B, whichreduces the by-production of isobutyraldehyde and/or isobutyric acid.

The nitrogen has substantially the same effect in reaction zone C.

To realize the abovementioned isobutane conversions, it is advantageousto carry out the isobutane dehydrogenation according to the invention inreaction zone A at a working pressure of from 0.3 to 3 bar. It isfurther advantageous to dilute the isobutane to be dehydrogenated withsteam. On the one hand, the heat capacity of the water allows theendothermic effect of the dehydrogenation to be partially compensatedfor and, on the other hand, dilution with steam reduces the reactant andproduct partial pressures, which has an advantageous effect on thedehydrogenation equilibrium location. In addition, the concomitant useof steam, as already mentioned, has an advantageous effect on theonstream time of the dehydrogenation catalyst. If required, molecularhydrogen may also be added as a further component. The molar ratio ofmolecular hydrogen to isobutane is generally ≦5. The molar ratio ofsteam to isobutane in the reaction zone A variant with comparatively lowisobutane conversion may accordingly be from ≧0 to 30, conveniently from0.1 to 2 and advantageously from 0.5 to 1. It has also provenadvantageous for a procedure with low isobutane conversion that only acomparatively low heat quantity is consumed in single reactor pass ofthe reaction gas and comparatively low temperatures are sufficient toachieve the conversion in single reactor pass.

According to the invention, it is therefore advantageous in the reactionzone A variant with comparatively low isobutane conversion to carry outthe isobutane dehydrogenation (quasi) adiabatically. In other words, thestarting reaction gas mixture will generally be heated to a temperatureof from 500 to 700° C. (for example by direct firing of the wallsurrounding it in a heater), or to from 550 to 650° C. Normally, asingle adiabatic pass through a catalyst bed will then be sufficient toachieve the desired conversion, and the reaction gas mixture will coolby from about 30° C. to 200° C. (depending on the conversion). Thepresence of steam as a heat carrier is also advantageous from the pointof view of an adiabatic method. The relatively low reaction temperatureallows relatively long onstream times of the catalyst bed used.

In principle, the reaction zone A variant according to the inventionhaving comparatively low isobutane conversion, whether performedadiabatically or isothermally, may also be carried out either in a fixedbed reactor or else in a moving bed or fluidized bed reactor.

Remarkably, a single shaft furnace reactor as the fixed bed reactor,through which the reaction gas mixture flows axially and/or radially, issufficient to realize this variant, particularly in adiabatic operation.

In the simplest case, this reactor is a single reaction tube whoseinternal diameter is from 0.1 to 10 m, possibly also from 0.5 to 5 m,where the fixed catalyst bed is mounted on a supporting device (forexample a grid). The reaction tube charged with catalyst, which may beheat-insulated in adiabatic operation, is flowed through axially by thehot, isobutane-containing reaction gas. The catalyst geometry may beeither spherical, extruded or annular. However, the catalyst mayadvantageously also be used in the abovementioned case in the form ofspall. To realize radial flow of the isobutane-containing reaction gas,the reactor may consist, for example, of two concentric cylindricalgrids disposed in a jacket and the catalyst bed may be arranged in theannular gap between them. In the adiabatic case, the jacket would inturn be thermally insulated.

Useful catalyst charges for the reaction zone A variant according to theinvention with comparatively low isobutane conversion in a single passare in particular the catalysts disclosed by DE-A 19 937 107, above allthose disclosed by way of example.

After a relatively long operation time, the abovementioned catalysts canbe regenerated, for example, in a simple manner by initially passingnitrogen, and/or steam-diluted air over the catalyst bed in firstregeneration stages at a temperature of from 300 to 900° C., frequentlyfrom 400 to 800° C., often from 500 to 700° C. The gas hourly spacevelocity of regeneration gas may be, for example, from 50 to 10 000 h⁻¹and the oxygen content of the regeneration gas may be from 0.5 to 20% byvolume.

In the further downstream regeneration stages, air may be used as theregeneration gas under otherwise identical regeneration conditions. Ithas proven advantageous from an application point of view to purge thecatalyst before regeneration with inert gas (for example N₂).

It is then generally recommended to regenerate further with puremolecular hydrogen or with molecular hydrogen diluted with inert gas(the hydrogen content should be ≧1% by volume) under otherwise identicalconditions. Frequently, it is advantageous to carry out the regenerationprocedure twice or more in succession.

The reaction zone A variant according to the invention withcomparatively low isobutane conversion (≦30 mol %) may in all cases beoperated at the same gas hourly space velocities (relating both to theoverall reaction gas and to the isobutane contained therein) as thevariants with high isobutane conversion (>30 mol %). This gas hourlyspace velocity of reaction gas may be, for example, from 100 to 10 000h⁻¹, frequently from 100 to 3000 h⁻¹, i.e. in many cases from 100 to2000 h⁻¹.

In a particularly elegant manner, the reaction zone A variant accordingto the invention with comparatively low isobutane conversion can berealized in a tray reactor.

This comprises more than one catalyst bed catalyzing the dehydrogenationin spatial succession. The catalyst bed number may be from 1 to 20,advantageously from 2 to 8 but also from 4 to 6. The catalyst beds arepreferably arranged in radial or axial succession. From an applicationpoint of view, it is advantageous to used the fixed catalyst bed type insuch a tray reactor.

In the simplest case, the fixed catalyst beds in a shaft furnace reactorare arranged axially or in the annular gaps of concentric cylindricalgrids.

Advantageously, the reaction gas mixture will be subjected tointermediate heating in the tray reactor on its way from one catalystbed to the next catalyst bed, for example by passing it over heatexchanger ribs heated by hot gases or by passing it through pipes heatedby hot combustion gases.

When the tray reactor is otherwise operated adiabatically, it issufficient for the desired isobutane conversions (≦30 mol %), inparticular when the catalysts described in DE-A 19 937 107, inparticular those of the exemplary embodiments, are used, to pass thereaction gas mixture into the dehydrogenation reactor preheated to atemperature of from 450 to 550° C. and to maintain it within thistemperature range inside the tray reactor. In other words, the entireisobutane dehydrogenation can thus be realized at very low temperatures,which is particularly advantageous for the onstream time of the fixedcatalyst beds.

It is even more beneficial to carry out the above-described intermediateheating in a direct way. To this end, a limited amount of molecularoxygen or a gas containing it is added to the reaction gas mixtureeither before it flows through the first catalyst bed and/or between thesubsequent catalyst beds. Depending on the dehydrogenation catalystused, a limited combustion of the hydrocarbons contained in the reactiongas mixture, any coke or coke-like compounds already deposited on thecatalyst surface and/or hydrogen formed in the course of the isobutanedehydrogenation and/or added to the reaction mixture is thus effected(it may also be advantageous from an application point of view to addcatalyst beds in the tray reactor which are charged with catalyst whichspecifically (selectively) catalyzes the combustion of hydrogen (and/orof hydrocarbon) (examples of useful catalysts include those of thedocuments U.S. Pat. Nos. 4,788,371, 4,886,928, 5,430,209, 55,530,171,5,527,979 and 5,563,314); for example, such catalyst beds could beaccommodated in the tray reactor in alternation to the beds containingthe dehydrogenation catalyst). The heat of reaction released thus allowsvirtually isothermal operation of the heterogeneously catalyzedisobutane dehydrogenation in a quasi-autothermal manner. As the selectedresidence time of the reaction gas in the catalyst bed is increased,isobutane dehydrogenation is thus possible at decreasing andsubstantially constant temperature which allows particularly longonstream times.

In general, oxygen feeding as described above should be carried out insuch a manner that the oxygen content of the reaction gas mixture, basedon the amount of isobutane and isobutene contained therein, is from 0.5to 10% by volume. Useful oxygen sources include both pure molecularoxygen and oxygen diluted with inert gas, for example CO, CO₂, N₂ ornoble gases, but in particular also air. The resulting combustion gasesgenerally have an additional dilution effect and thus support theheterogeneously catalyzed isobutane dehydrogenation. The dehydrogenationtemperature in the tray reactor in the process according to theinvention is generally from 400 to 800° C., and the pressure isgenerally from 0.2 to 10 bar, preferably from 0.5 to 4 bar and morepreferably from 1 to 2 bar. The gas hourly space velocity is generallyfrom 500 to 2000 h⁻¹, and in high-load operation even up to 16 000 h⁻¹,regularly from 4000 to 16 000 h⁻¹.

The isothermicity of the heterogeneously catalyzed isobutanedehydrogenation can be further improved by incorporating closedinternals (for example tubular) which have been evacuated before fillingin the spaces between the catalyst beds in the tray reactor. It will beappreciated that such internals may also be placed in each catalyst bed.These internals contain suitable solids or liquids which evaporate ormelt above a certain temperature, thereby consuming heat, and, when thetemperature falls below this value, condense again and thereby releaseheat.

Another possible method of heating the reaction gas mixture to therequired temperature in reaction zone A of the process according to theinvention consists in combusting a portion of the isobutane and/or H₂contained therein using molecular oxygen (for example over specificcombustion catalysts, for example by simply passing them over and/orthrough) and to effect the heating to the desired reaction temperatureby means of the heat of combustion released in this manner. Theresulting combustion products such as CO₂, H₂O and also any N₂accompanying the molecular oxygen required for the combustionadvantageously take on the role of inert diluent gases.

It will be appreciated that reaction zone A according to the inventioncan also be realized in a jet pump circulation reactor as described byDE-A 10 211 275. Quite generally, all dehydrogenation variants describedin DE-A 10 211 275 are usable in reaction zone A according to theinvention.

It is essential to the invention that the isobutane used in reactionzone A is not pure isobutane. Rather, the isobutane used may comprise upto 50% by volume of other gases, for example, ethane, methane, ethylene,n-butanes, n-butenes, propyne, acetylene, propane, propene, H₂S, SO₂,pentanes, etc. Advantageously, the crude isobutane to be used comprisesat least 60% by volume, advantageously at least 70% by volume,preferably at least 80% by volume, more preferably at least 90% byvolume and most preferably at least 95% by volume, of isobutane. Inparticular, a mixture of isobutane, isobutene and cycle gas arising fromremovals from the product gas mixture A may also be used for thereaction zone A according to the invention.

The product gas mixture A leaving reaction zone A in the processaccording to the invention comprises at least the components isobutaneand isobutene, and also generally molecular hydrogen. Furthermore, itwill generally also comprise gases from the group consisting of N₂, H₂O,methane, ethane, ethylene, propane, propene, CO and CO₂ and alsopossibly O₂.

The mixture will generally be at a pressure of from 0.3 to 10 bar andfrequently a temperature of from 450 to 500° C.

Quite generally, reactors having passivated interior walls are used forreaction zone A according to the invention. The passivation may beeffected, for example, by applying sintered aluminum oxide to theinterior wall before dehydrogenation or by using a silicon-containingsteel as the reactor material which forms a passivating SiO₂ layer onthe surface under the reaction conditions. However, passivation may alsobe achieved in situ by adding small quantities of passivatingauxiliaries (for example sulfides) to the reaction zone A charging gasmixture.

The removal of components other than isobutane and isobutene from theproduct gas mixture A which is essential according to the invention maybe carried out in different ways. For instance, separating processes maybe applied in succession, each of which is able to remove onlyindividual components.

For example, the hydrogen may be removed by passing the product gasmixture A, optionally after it has been cooled in an indirect heatexchanger (advantageously, the heat removed is used to heat one of thefeed gases required for the process according to the invention), over amembrane, generally configured as a tube, which is only permeable towardmolecular hydrogen. The molecular hydrogen individually removed in thismanner may, if required, be at least partially added to reactor zone Aor utilized in another way. In the simplest case, it may be combusted infuel cells.

Steam contained in the reaction gas mixture A may be individuallyremoved simply in a condensation stage and, if required, recycled atleast partially into the reaction zone A.

Removal of hydrogen and steam is possible, for example, by convertinghydrogen contained in the reaction gas mixture to steam by selectiveheterogeneously catalyzed combustion using oxygen over suitablecatalysts and then removing it by condensation. When any oxygen in thereaction gas mixture A is used for the abovementioned selectivecombustion, molecular oxygen contained in the product gas mixture A maybe removed at the same time in the abovementioned manner. The disclosurecontent of U.S. Pat. Nos. 4,788,371, 4,886,928, 5,430,209, 5,530,171,55,279,979 and 5,563,314 relates to catalysts suitable for such aselective combustion.

In a corresponding manner, CO contained in the reaction gas mixture Acan be selectively combusted to CO₂ using suitable catalysts and,together with CO₂ already contained in the reaction gas mixture A, beremoved by scrubbing with a basic liquid. Examples of such basic liquidsinclude aqueous alkali metal hydroxide solutions, aqueous ammoniasolutions and organic amines. Nitrogen remaining in the mixture withisobutane and isobutene may, if required, be removed from them bycondensation of the hydrocarbons.

A simple method of removing substantially all components of the productgas mixture A other than isobutane and isobutene consists in contacting(for example by simply passing through) the preferably cooled(preferably to temperatures of from 10 to 70° C.) product gas mixture A,for example at a pressure of from 0.1 to 50 bar and a temperature offrom 0 to 100° C., with a (preferably high-boiling) organic solvent(preferably hydrophobic) which preferentially absorbs isobutane andisobutene. Subsequent desorption, for example by heating,depressurization-evaporation (flashing) and/or distillation(rectification) or stripping using a gas which is inert in relation toreaction zone B (for example nitrogen and/or steam) and/or molecularoxygen or mixtures of inert gases and molecular oxygen (for example air)recovers the isobutane and isobutene in the mixture which are used tocharge reaction zone B.

When the stripping gas used is air or an oxygen-nitrogen mixture wherethe oxygen content is above 10% by volume it may be sensible to add agas before or during the stripping process which reduces the explosionrange. Particularly suitable gases for this purpose have a heat capacityof ≧29 J/mol·K (based on 25° C. and 1 atm). For example, isobutane maybe used as such a gas.

The absorption offgas containing the molecular hydrogen can, forexample, again be subjected to membrane separation and then, ifrequired, the hydrogen removed can be used in reaction zone A. Theresidual gas remaining after the hydrogen removal may, if required, beused as a diluent gas in reaction zones A, B and/or C. The boiling pointof the organic absorbent should preferably be ≧100° C., more preferably≧180° C. The absorption may be carried out using columns or else inrotary absorbers. Operation may be effected in cocurrent orcountercurrent. Examples of useful absorption columns include traycolumns (having bubble cap, centrifugal or sieve trays), columns havingstructured packings (for example sheet metal packings having a specificsurface area of from 100 to 500 m²/m³, for example Mellapak® 250 Y) andrandomly packed columns (for example packed with Raschig shaped bodies).It will be appreciated that trickle and spray towers, graphite blockabsorbers, surface absorbers such as thick film and thin film absorbersand also rotary columns, plate scrubbers, cross-spray scrubbers androtary scrubbers may also be considered.

It is advantageous according to the invention when the organic absorbentto be used on the one hand fulfills the boiling point recommendationalready given but on the other hand at the same time does not have toohigh a molecular weight. Advantageously, the molecular weight of theabsorbent is ≦300 g/mol.

Examples of absorbents suitable according to the invention includerelatively nonpolar organic solvents which preferably contain no polargroups having any external effect. Examples thereof include aliphatic(for example C₈- to C₁₈-alkenes) or aromatic hydrocarbons, for examplemiddle oil fractions from paraffin distillation, or ethers having bulkygroups on the oxygen atom, or mixtures thereof, to which a polarsolvent, for example the 1,2-dimethyl phthalates disclosed in DE-A 4 308087, may be added. Further suitable absorbents include esters of benzoicacid and phthalic acid with straight-chain alkanols containing from 1 to8 carbon atoms such as n-butyl benzoate, methyl benzoate, ethylbenzoate, dimethyl phthalate and diethyl phthalate, and also heatcarrier oils such as diphenyl or diphenyl ether and mixtures of diphenyland diphenyl ether or chlorine derivatives thereof and triarylalkenes,for example 4-methyl-4′-benzyldiphenylmethane and its isomers2-methyl-2′-benzyldiphenylmethane, 2-methyl-4′-benzyldiphenylmethane and4-methyl-2′-benzyldiphenylmethane and mixtures of such isomers. A usefulabsorbent is a solvent mixture of diphenyl and diphenyl ether,preferably in the azeotropic composition, in particular of about 25% byweight of diphenyl (biphenyl) and about 75% of diphenyl ether, forexample the commercially obtainable Diphyl. Frequently, this solventmixture comprises a solvent such as dimethyl phthalate in an amount offrom 0.1 to 25% by weight, based on the entire solvent mixture. Otherpossible absorbents include octanes, nonanes, decanes, undecanes,dodecanes, tridecanes, tetradecanes, pentadecanes, hexadecanes,heptadecanes and octadecanes.

The paraffin oils having from 8 to 10 carbon atoms described in DE-A 3313 573 are likewise suitable. Examples of suitable trade productsinclude the products sold by Haltermann including the Halpasols®i, forexample Halpasol 250/340 i and Halpasol 250/275 i, and also printing inkdistillates under the names PKWF and Printosol.

In order to minimize absorbent losses in removing the isobutene from theabsorbate, both the gas phase resulting from the stripping and also arising gas phase in the distillation or rectification may be scrubbed incountercurrent using water. The scrubbing water used may be steam afterits condensation which was contained in the offgas or absorption.Otherwise, the absorptive removal can be carried out as described inWO-0196271 using the example of propane/propene.

An alternative possibility for removing the components other thanisobutane and isobutene from product gas mixture A is offered byfractional distillation (rectification). Advantageously, a fractionalpressure distillation is carried out at low temperatures. The pressureto be applied may be, for example, from 10 to 100 bar. Usefulrectification columns include randomly packed columns, tray columns orcolumns with structured packing. Useful tray columns include thosehaving dual-flow trays, bubble cap trays or valve trays. Tworectification columns, for example, may be attached in series in asimple manner. In the first column, the components having higher boilingpoints than isobutane and isobutene are removed as the bottom product.In the second column, isobutane and isobutene may be removed overheadfrom lower-boiling components.

After the removal required according to the invention of at least 80 mol% of the components other than isobutane and isobutene contained in theproduct gas mixture A, the resulting product gas mixture A′ may be usedto charge reaction zone B. If required, the product gas mixture A′ maybe brought to the reaction temperature required in reaction zone A′ byindirect heat exchange.

The methacrolein-containing product gas mixture B formed in reactionzone B is then used without preceding removal of components containedtherein to charge a reaction zone C.

The basis for the configuration of the second part of the processaccording to the invention in two spatially successive reaction zones Band C is the fact that the heterogeneously catalyzed gas phase partialoxidation of isobutene with molecular oxygen to methacrylic acidproceeds in two successive steps along the reaction coordinate, of whichthe first leads to methacrolein and the second from methacrolein tomethacrylic acid.

In each of the two reaction zones B and C, the oxidic catalyst to beused may be optimized in the same manner as the reaction conditions. Forinstance, for the first oxidation zone, the reaction zone B(isobutene→methacrolein), preference is generally given to a catalystbased on multimetal oxides comprising the element combination Mo—Bi—Fe,while for the second oxidation zone, the reaction zone C(methacrolein→methacrylic acid), preference is normally given tocatalysts based on multimetal oxides based on the element combinationMo—P (in particular the heteropolyacids).

Examples of multimetal oxide catalysts suitable for reaction zone B aredisclosed by U.S. Pat. Nos. 4,954,650, 5,166,119, DE-A 10 121 592(multimetal oxide compositions of the formulae I, II and III in the sameDE-A), DE-A 10 046 957 (multimetal oxide compositions of the formulae Iand II in the same DE-A), DE-A 10 101 695 (multimetal oxide compositionsof the formulae I, II and III in the same DE-A), DE-A 10 063 162(multimetal oxide compositions of the formula I in the same DE-A), DE-A10 059 713 (multimetal oxide compositions of the formula I in the sameDE-A) and DE-A 10 049 873 (multimetal oxide compositions of the formulaI in the same DE-A).

A variety of multimetal oxide compositions suitable as catalysts for thereaction zone B can be subsumed by the general formula IMo₁₂Bi_(a)Fe_(b)X_(c) ¹X_(d) ²X_(e) ³X_(f) ⁴O_(n)  (I)where the variables are defined as follows:

-   X¹=nickel and/or cobalt,-   X²=alkali metal, thallium and/or an alkaline earth metal,-   X³=zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead    and/or tungsten,-   X⁴=silicon, aluminum, titanium and/or zirkonium,-   a=from 0.5 to 5,-   b=from 0.01 to 5, preferably from 2 to 4,-   c=from 0 to 10, preferably from 3 to 10,-   d=from 0 to 2, preferably from 0.02 to 2,-   e=from 0 to 8, preferably from 0 to 5,-   f=from 0 to 10 and-   n=a number which is determined by the valency and frequency of the    elements other than oxygen in I.

The compositions are obtainable in a manner known per se (cf., forexample, DE-A 10 121 592) and are customarily used as such shaped intospheres, rings, cylinders or else in the form of coated catalysts, i.e.preshaped inert support particles coated with the active composition.

Examples of suitable unsupported catalyst geometries include solidcylinders and hollow cylinders having an external diameter and a lengthof from 2 to 10 mm. In the case of the hollow cylinders, a wallthickness of from 1 to 3 mm is advantageous. It will be appreciated thatthe unsupported catalyst may also have spherical geometry and the spherediameter may be from 2 to 10 mm. Useful coated catalyst geometries arelikewise disclosed by DE-A 10 121 592.

According to the invention, further multimetal oxide compositions usefulas catalysts for reaction zone B are compositions of the general formulaII[Y¹ _(a′)Y² _(b′)O_(x′)]_(p)[Y³ _(c′)Y⁴ _(d′)Y⁵ _(e′)Y⁶ _(f′)Y⁷ _(g′)Y²_(h′)O_(y′)]_(q)  (II)where the variables are defined as follows:

-   Y¹=only bismuth or bismuth and at least one of the elements    tellurium, antimony, tin and copper,-   Y²=molybdenum and/or tungsten,-   Y³=an alkali metal, thallium and/or samarium,-   Y⁴=an alkaline earth metal, nickel, cobalt, copper, manganese, zinc,    tin, cadmium and/or mercury,-   Y⁵=iron or iron and at least one of the elemenents chromium, cerium    and vanadium,-   Y⁶=phosphorus, arsenic, boron and/or antimony,-   Y⁷=a rare earth metal, titanium, zirconium, niobium, tantalum,    rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium,    indium, silicon, germanium, lead, thorium and/or uranium,-   a′=from 0.01 to 8,-   b′=from 0.1 to 30,-   c′=from 0 to 4,-   d′=from 0 to 20,-   e′=from 0 to 20,-   f′=from 0 to 6,-   g′=from 0 to 15,-   h′=from 8 to 16,-   x′,y′=numbers which are determined by the valency and frequency of    elements other than oxygen in II and-   p,q=numbers whose ratio p/q is from 0.1 to 10.

In favorable cases, the multimetal oxide compositions II comprisethree-dimensional regions which are delimited from their localenvironment owing to their different composition from their localenvironment and of the chemical composition Y¹ _(a′)Y² _(b′)O_(x′) andwhose maximum diameter (longest line connecting two points on thesurface (boundary layer) of the region and passing through the mainfocus of the region) is from 1 nm to 100 μm, frequently from 10 nm to500 nm or from 1 μm to 50 or 25 mm.

With regard to the shaping, the remarks made on multimetal oxidecompositions I catalysts apply to multimetal oxide compositions IIcatalyts.

The preparation of multimetal oxide compositions II catalysts isdescribed, for example, by EP-A 575897, DE-A 19855913, DE-A 10 046 957and DE-A 10 121 592.

Reaction zone B is most easily realized by a tube bundle reactor whosecatalyst tubes are charged with catalyst. The configuration may beentirely similar to the teaching of EP-A 911313, EP-A 979813, EP-A990636 and DE-A 2830765 for the partial oxidation of propylene toacrolein. Otherwise, reaction zone B may be configured as taught in U.S.Pat. Nos. 4,954,650 and 5,166,119.

The reaction temperature is generally from 250 to 450° C. The reactionpressure is advantageously from 0.5 to 5, frequently from 1 to 3, bar.The gas hourly space velocity (1 at STP/l·h) on the oxidation catalystsis frequently from 1500 to 2500 h⁻¹ or 4000 h⁻¹.

In principle, reaction zone B may also be configured as described forsimilar reactions, for example, in DE-A 19837517, DE-A 19910506, DE-A19910508 and also DE-A 19837519.

Customarily, the external heating, if appropriate in multizone reactorsystems, is adjusted in a manner known per se to the specific reactiongas mixture composition and also catalyst charge.

The molecular oxygen required in the reaction zone B necessary for theinvention is normally added in advance in its entirety to the charginggas mixture of reaction zone B.

Normally, a molar isobutene:molecular oxygen ratio in the charging gasfor reaction zone B of from 1:1 to 3, frequently from 1:1.5 to 2.5, isset.

An excess (based on the stoichiometry of the gas phase partialoxidation) of molecular oxygen generally has an advantageous effect onthe kinetics of the gas phase oxidation in reaction zone B. In contrastto the conditions in the reaction zone A to be applied according to theinvention, the thermodynamic ratios in reaction zone B are substantiallynot influenced by the molar reactant ratio, since the heterogeneouslycatalyzed gas phase partial oxidation of isobutene to methacrolein isunder kinetic control.

In principle, it is therefore also possible, for example, to initiallycharge the isobutene into reaction zone B in a molar excess relative tothe molecular oxygen. In this case, the excess isobutene actuallyassumes the role of a diluent gas.

A useful source for the molecular oxygen required overall in reactionzone B which is normally admixed with product gas mixture A′ before itis used to charge reaction zone B is in particular oxygen diluted withmolecular nitrogen. Advantageously, air will be used as the oxygensource at least to cover part of the need for molecular oxygen, sincethe nitrogen also to be used in reaction zone B may be introduced intothe reaction system in this manner in a very simple way.

However, a portion of the molecular oxygen required overall in reactionzone B may also already be contained in the product gas mixture A′introduced into reaction zone B. However, preference is given to no moreoxygen being contained therein. Further useful oxygen sources usable inthe reaction zone include molecular oxygen diluted with inert gases suchas CO₂, CO, noble gases, N₂ and/or saturated hydrocarbons. An example ofsuch an oxygen source is a cycle gas diverted from the process accordingto the invention and recycled into reaction zone B.

In the process according to the invention, the source for molecularoxygen in the downstream reaction zone B, apart from any molecularoxygen already contained in the product gas mixture A′, isadvantageously at least partially and preferably exclusively air.

The metering of cold air at room temperature into the hot product gasmixture A′ in the process according to the invention may, if required,be used to cool the product gas mixture A′ on its way into reaction zoneB.

The product gas mixture B leaving reaction zone B to be used accordingto the invention is generally substantially composed of the targetproduct methacrolein or a mixture thereof with methacrylic acid,unconverted molecular oxygen, isobutane, unconverted isobutene (themolar conversion of isobutene in the reaction zone B according to theinvention is preferably ≧96 or ≧97 mol %, more preferably ≧98 mol % andmost preferably ≧99 mol %), molecular nitrogen (optionally molecularhydrogen), steam formed as a by-product and/or used as a diluent gas,carbon oxides as a by-product and/or used as a diluent gas, and alsosmall amounts of other lower aldehydes, hydrocarbons and other inertdiluent gases. However, the isobutyraldehyde and isobutyric acidcontents are minimized in accordance with the invention. It is essentialto the invention that the molar conversion of isobutene in the reactionzone B is ≧95 mol %.

Advantageous catalysts for reaction zone C are disclosed, for example,by DE-A 4405060, U.S. Pat. Nos. 5,166,119, 5,153,162, 4,954,650,4,558,028 and DE-A 19 815 279. These patents also teach the use of suchcatalysts. As well as molybdenum and phosphorus, they customarilycomprise metallic and transition metallic elements, in particularcopper, vanadium, arsenic, antimony, cesium and also potassium (cf. DE-A4329907, DE-A 2610249, JP-A 7/185354).

DE-A 19 922 113 suggests multimetal oxide compositions of the generalformula III[A]_(p)[B]_(q)  (III)where the variables are defined as follows:

-   A: MO₁₂X_(a) ¹X_(b) ²X_(c) ³X_(d) ⁴X_(e) ⁵O_(x)-   B: Mo_(f)X⁶ _(g)X⁷ _(h)O_(y)-   x¹=H, of which up to 97 mol % may be replaced by ammonium, K, Rb    and/or Cs,-   X²=V, Nb, Ta, W and/or Re,-   X³=B, Si, P, Ge, As and/or Sb,-   X⁴=Cr, Mn, Fe, Co, Ni, Cu, Zn, Mg, Ca, Sr and/or Ba,-   X⁵=S,-   X⁶=Cu, Fe, Co, Ni, Zn, Cd, Mn, Mg, Ca, Sr and/or Ba,-   X⁷=Nb, Ta and/or Sb,-   a=from 1 to 3,-   b=from 0.1 to 2,-   c=from 0 to 5,-   d=from 0 to 1,-   e=from 0 to 1,-   f=from 0 to 2,-   g=from 0.5 to 1.5,-   h=from 2 to 4,-   x,y=numbers which are determined by the valency and frequency of the    elements other than oxygen in (I),    -   p=1 and q=0 or-   p, q are integers other than zero whose ratio p/q is from 160:1 to    1:1 and the standard deviation of the stoichiometric coefficients a    of the X¹ variables of individual crystallites within the component    A of the multimetal oxide composition is less than 0.40, preferably    less than 0.20, in particular less than 0.11.

The compositions preferably comprise the fraction [A]_(p) in the form ofthree-dimensional regions A of chemical composition A which aredelimited from their local environment owing to their different chemicalcomposition and the fraction [B]_(q) in the form of three-dimensionalregions B of chemical composition B which are delimited from their localenvironment owing to their different chemical composition from theirlocal environment, and the regions A and B are distributed relative toeach other as in a finely divided mixture of A and B.

DE-A 4405060 recommends similar multimetal oxide compositions for areaction zone C. Like the multimetal oxide catalysts for reaction zoneB, the multimetal oxide catalysts for reaction zone C are customarilyused as such shaped into spheres, rings or cylinders or else in the formof coated catalysts, i.e. preformed inert support particles coated withthe active composition.

Examples of suitable unsupported catalyst geometries include solidcylinders and hollow cylinders having an external diameter and a lengthof from 2 to 10 mm. In the case of the hollow cylinders, a wallthickness of from 1 to 3 mm is advantageous. It will be appreciated thatthe unsupported catalyst may also have spherical geometry and the spherediameter may be from 2 to 10 mm. Examples of useful coated catalystgeometries are those disclosed by DE-A 10 121 592.

The simplest way of realizing reaction zone C, as in the case ofreaction zone B, is a tube bundle reactor, and so all remarks made inrelation to the tube bundle reactor of reaction zone B are alsosimilarly valid for a tube bundle reactor of reaction zone C.

With regard to the flow of reaction gas and heating medium (for examplesalt bath), the tube bundle reactors recommended for both reaction zoneB and reaction zone C may be operated either in cocurrent or else incountercurrent. It will be appreciated that crosscurrent flows may alsobe superimposed. A meandering flow of the temperature medium around thecatalyst tubes is particularly advantageous and, viewed over thereactor, may again be in cocurrent or in countercurrent to the reactiongas mixture.

A particularly simple way of realizing the reaction zones B and C isaccordingly a tube bundle reactor within which the catalyst chargechanges correspondingly along the individual catalyst tubes. The chargeof the catalyst tubes with catalyst may optionally be interrupted by aninert bed (EP-A 911313, EP-A 979813, EP-A 990636 and DE-A 2830765 teachsuch a procedure in an equivalent manner using the example of partialoxidation of propylene to acrylic acid). In the case of this way ofrealization, the molecular oxygen required in reaction zone C alreadyhas to be contained in the charging gas mixture for reaction zone B.

However, preference is given to realizing the two reaction zones B and Cin the form of two tube bundle systems connected in series. These mayoptionally be in one reactor and one tube bundle may be connected to theother tube bundle by a bed (advantageously accessible on foot) of inertmaterial which is not accommodated in the catalyst tubes. While thecatalyst tubes are generally purged through by a heat carrier, this doesnot reach an inert bed installed as described above. However, the twocatalyst tube bundles will advantageously be accommodated in spatiallyseparated reactors. There may be an intermediate cooler between the twotube bundle reactors in order to reduce any continued methacroleincombustion in the product gas mixture which leaves reaction zone B.Instead of tube bundle reactors, plate heat exchanger reactors havingsalt and/or evaporative cooling, as described, for example, by DE-A19929487 and DE-A 19952964, may also be used.

The reaction temperature in reaction zone C is generally from 230 to350° C., frequently from 250 to 320° C. The reaction pressure inreaction zone C is advantageously from 0.5 to 5, frequently from 1 to 3or 2, bar. The gas hourly space velocity (1 at STP/l·h) on the oxidationcatalysts of reaction C of charging gas mixture is frequently from 1000to 2500 h⁻¹ or to 4000 h⁻¹.

As already mentioned, the molecular oxygen required overall as anoxidizing agent in reaction zone C may already be added in advance tothe charging gas mixture of reaction zone B in its entirety. However, itwill be appreciated that supplementation with oxygen may also beeffected after reaction zone B. The latter possibility is used inparticular when the two reaction zones B and C are realized in the formof two tube bundle systems in series.

Since the molecular oxygen used in reaction zone B is also a componentof the charging gas mixture of reaction zone C, such oxygensupplementation may be carried out by means of pure molecular oxygen. Afurther oxygen source usable for such supplementation purposes ismolecular oxygen diluted with inert gases such as CO₂, CO, noble gases,N₂ and/or saturated hydrocarbons. An example of such an oxygen sourcemay be a cycle gas diverted from the process according to the inventionand recycled into reaction zone C. According to the invention,preference is given to using air for such oxygen supplementation.

Even in reaction zone C, an excess (based on the reaction stoichiometry)of molecular oxygen generally has an advantageous effect on the kineticsof the gas phase oxidation. Preference is given to setting a molarmethacrolein:molecular oxygen ratio in reaction zone C of from 1:1 to 3,frequently from 1:1.5 to 2.5.

Frequently, the process according to the invention is carried out insuch a manner that at least 50 mol %, preferably at least 60 mol %, ofthe total amount of molecular oxygen in the product gas mixture C whichhas been introduced in the various reaction zones has been converted.

Frequently, the process according to the invention in reaction zone Cwill be performed at a molar methacrolein:molecularoxygen:steam:isobutane:molecular nitrogen:other diluent gas ratio of2-5:5-15:0-20:5-25:20-80:0-6.

However, reaction zones B and C may in principle also be formallycombined into a single reaction zone. In this case, the two reactionsteps (isobutene→methacrolein; methacrolein→methacrylic acid) areeffected in an oxidation reactor which is charged with a catalyst whichis able to catalyze the reaction of both successive reaction steps.

The metering of cold (at ambient temperature) air into the hot productgas mixture B in the process according to the invention may also be usedas a direct way of cooling the product gas mixture B before it is usedto charge reaction zone C.

The product gas mixture C leaving reaction zone C is generally composedsubstantially of methacrylic acid, methacrolein, unconverted molecularoxygen, isobutane, molecular nitrogen, steam formed as a by-productand/or used as a diluent gas, (optionally molecular hydrogen), carbonoxides formed as a by-product and/or used as a diluent gas, and alsosmall quantities of other lower aldehydes, hydrocarbons and other inertdiluent gases. Its isobutyraldehyde and isobutyric acid contents areminimized in accordance with the invention.

The methacrylic acid may be removed from the product gas mixture C in amanner known per se.

For example, product gas mixture C (which may have an exit temperatureof, for example, 220° C.) may first be cooled by direct contact with a10% by weight aqueous solution of methacrylic acid which may bepolymerization-inhibited, for example, by the addition of small amountsof hydroquinone monomethyl ether (MEHQ) and have a temperature of 80° C.To this end, the aqueous methacrylic acid solution is sprayed into theproduct gas mixture C in an apparatus substantially free of internalsand passed in cocurrent with it. Mist which forms may be separated fromthe gas phase in two Venturi precipitators. Afterwards, the cooledproduct gas mixture is then passed into the bottom of an absorptioncolumn, for example a randomly packed column. At the top of the column,water which contains the polymerization inhibitors dissolved is added asthe absorbing liquid in countercurrent to the rising gas. The toptemperature may be, for example, 63° C. and the bottom temperature, forexample, 70° C.

Together with medium- and high-boiling by-products such as acetic,propionic, acrylic, maleic, fumaric, citraconic and formic acid and alsoformaldehyde and any isobutyraldehyde and isobutyric acid formed, themethacrylic acid is removed in the absorber from the gas phase into theaqueous phase.

The methacrylic acid can be removed extractively from the from 10 to 20%by weight aqueous methacrylic acid solution withdrawn from the bottom ofthe absorber using suitable extractants, for example ethylhexanoic acid,and subsequently isolated rectificatively.

The residual gas leaving the absorber at the top generally comprisesisobutane, isobutene, methacrolein, O₂, (possibly H₂), N₂, CO, CO₂, H₂O,noble gases and also other lower aldehydes and hydrocarbons.

The methacrolein can be removed therefrom by means of subsequentscrubbing with water and freed again from the scrubbing water bystripping using air and recycled with the air into reaction zone C.

According to the invention, the residual gas remaining after themethacrylic acid removal which normally comprises unconverted isobutaneand isobutene will preferably be recycled as such into the reaction zoneA. However, it is also possible in the process according to theinvention to first substantially remove the isobutane and isobutenegenerally contained in the residual gas remaining after the methacrylicacid removal from other gases such as O₂ (possibly H₂), N₂, CO, CO₂,noble gases, etc. contained therein by absorption with subsequentdesorption and/or stripping and also absorbent reuse in a high-boilinghydrophobic organic solvent and then to recycle them into reaction zoneA. If required, the remaining other gases from the mixture may berecycled as diluent gas into reaction zones B and/or C. However, theymay also be discharged, for example incinerated.

In general, solvents useful as absorbents for the abovementioned purposeinclude relatively nonpolar organic solvents, for example aliphatichydrocarbons, which preferably have no external polar groups, and alsoaromatic hydrocarbons. In general, it is desirable that the absorbentshave a very high boiling point and at the same time very high solubilityfor isobutane and/or isobutene and very low solubility for the otherresidual gas components.

Examples of useful absorbents include aliphatic hydrocarbons, forexample C₈-C₂₀-alkanes or -alkenes, or aromatic hydrocarbons, forexample middle oil fractions from paraffin distillation, or ethershaving bulky groups on the oxygen atom, or mixtures thereof, to which apolar solvent, for example the 1,2-dimethyl phthalate disclosed in DE-A4308087. Further suitable absorbents include esters of benzoic acid andphthalic acid with straight-chain alkanols containing from 1 to 8 carbonatoms such as n-butyl benzoate, methyl benzoate, ethyl benzoate,dimethyl phthalate and diethyl phthalate, and also heat carrier oilssuch as diphenyl or diphenyl ether and mixtures of diphenyl and diphenylether or chlorine derivatives thereof and triarylalkenes, for example4-methyl-4′-benzyldiphenylmethane and its isomers2-methyl-2′-benzyldiphenylmethane, 2-methyl-4′-benzyldiphenylmethane and4-methyl-2′-benzyldiphenylmethane and mixtures of such isomers. A usefulabsorbent is also a solvent mixture of diphenyl and diphenyl ether,preferably in the azeotropic composition, in particular of about 25% byweight of diphenyl (biphenyl) and about 75% by weight of diphenyl ether,for example the commercially obtainable Diphyl. Frequently, this solventmixture comprises a solvent such as dimethyl phthalate in an amount offrom 0.1 to 25% by weight, based on the entire solvent mixture.Particularly useful absorbents also include octanes, nonanes, decanes,undecanes, dodecanes, tridecanes, tetradecanes, pentadecanes,hexadecanes, heptadecanes and octadecanes, and tetradecanes inparticular have proven particularly useful. It is advantageous when theabsorbent used on the one hand attains the abovementioned boiling pointand on the other hand at the same time does not have too high amolecular weight. Advantageously, the molecular weight of the absorbentis ≦300 g/mol. The paraffin oils having from 8 to 10 carbon atomsdescribed in DE-A 3313573 are likewise suitable. Examples of usefultrade products include the products sold by Haltermann includingHalpasols®i, for example Halpasol 250/340i and Halpasol 250/275i, andalso printing ink distillates sold as PKWF and Printosol.

The performance of the absorption is subject to no particularrestrictions. All processes and conditions familiar to those skilled inthe art may be used. Preference is given to contacting the gas mixturewith the absorbent at a pressure of from 1 to 50 bar, preferably from 2to 20 bar, more preferably from 5 to 10 bar, and a temperature of from 0to 100° C., in particular from 30 to 50° C. The absorption may becarried out in absorption columns, for example tray columns havingbubble cap and/or sieve trays, columns having structured packings orrandomly packed columns, in trickle and spray towers, graphite blockabsorbers, surface absorbers such as thick film and thin film absorbersand also plate scrubbers, cross-spray scrubbers and rotary scrubbers. Itmay also be advantageous to carry out the absorption in a bubble columnwith or without internals.

The isobutane and/or isobutene may be removed from the absorbate bystripping, depressurization-evaporation (flashing) and/or distillation(rectification).

Gases suitable for stripping are in particular those which may berecycled into reaction zone A together with the isobutane and isobutene.

Such gases include nitrogen, air, oxygen, oxygen/nitrogen mixtures,isobutane and steam. When air or oxygen/nitrogen mixtures are used wherethe oxygen content is above 10% by volume, it may be sensible to add agas which reduces the explosion range before or during the strippingprocess. Particularly suitable gases for this purpose have a heatcapacity of ≧29 J/mol-K (based on 25° C. and 1 atm). For example,isobutane may also be used as such a gas.

The isobutane and/or isobutene may also be removed from the absorbate bya distillation. In order to minimize absorbent losses, both the gasphase resulting from the stripping and a rising gas phase resulting fromdistillation may be scrubbed in countercurrent using water. Thescrubbing water used may be condensed steam contained in the residualgas.

Otherwise, the procedure may be as described in WO-0196271 using theexample of propane/propene.

Further possible methods of removing isobutane and/or isobutene from theresidual gas are adsorption, rectification and partial condensation.Preference is given to carrying out a fractional pressure distillationat low temperatures. The pressure to be applied may be, for example,from 10 to 100 bar. Useful rectification columns include randomly packedcolumns, tray columns or columns having structured packing. Useful traycolumns include those having dual-flow trays, bubble cap trays or valvetrays. The reflux ratio may be, for example, from 1 to 10. Examples ofother possible separating methods include pressure extraction, pressureswing adsorption, pressure scrubbing and partial condensation. For thepurposes of a fractional distillation, the separating line may, forexample, be defined in such a manner that substantially all of thosecomponents whose boiling point is lower than the boiling point ofisobutene are removed at the top of the rectification column. Thesecomponents will primarily be the carbon oxides CO and CO₂ and alsounconverted oxygen and N₂. Steam may be recycled together with isobutaneand isobutene into reaction zone A.

A more comprehensive description of the above-outlined removal ofmethacrylic acid and methacrolein from a product gas mixture such asproduct gas mixture C can be found in EP-B 297445

However, it will be appreciated that the separating processes of U.S.Pat. Nos. 4,925,981 and 4,554,054 may also be used for this purpose.

It is common to all these processes that a residual gas comprisingunconverted isobutane and generally isobutene remains after themethacrylic acid removal. According to the invention, preference isgiven to recycling this, as already mentioned, as such into reactionzone A.

It will be appreciated that the process according to the invention mayalso be carried out by recycling only a portion of the residual gasunchanged into reaction zone A and removing isobutane and isobutene inthe mixture only from the remaining portion and likewise recycling theminto reaction zone A.

If the gas containing isobutane and isobutene to be recycled intoreaction zone A still contains carbon monoxide, this may becatalytically selectively combusted to CO₂ before (or after) entry intoreaction zone A. The heat of reaction released may be used to heat tothe dehydrogenation temperature.

Catalytic postcombustion of CO contained in the residual gas to CO₂ mayalso be recommendable when removal of the carbon oxides from theresidual gas is sought before it is recycled into reaction zone A (oranother zone), because CO₂ can be comparatively easily removed (forexample by scrubbing with a basic liquid).

When dehydrogenation catalysts are used which are sensitive towardoxygen or oxygen-containing compounds, these oxygenates will be removedfrom the residual gas before recycling of the residual gas into reactionzone A. This is unnecessary for the catalysts particularly recommendedfor the catalytic dehydrogenation in this document.

It will be appreciated that isobutane and/or isobutene removed orresidual gas containing these gases may also be utilized for purposesother than recycling to reaction zone A (for example for preparingisobutanol or for combustion for the purposes of energy generation).

The advantage according to the invention of reduced isobutyraldehyde andisobutyric acid by-production is substantially independent of themultimetal oxide catalysts used in reaction zones B and C. Preference isgiven to using those multimetal oxide catalysts which are explicitlymentioned in this document. This advantage is also substantiallyindependent of whether the volume-specific catalyst activity in reactionzones B and C is kept constant or increases or decreases along thereaction coordinate.

In general, operation is effected in reaction zone C of the processaccording to the invention at a molar methacrolein oxygen:steam:inertgas ratio of (2-5):(5-15):(0-20):(5-25):(20-80):(0-6), more preferablyof (3-4):(6-10):(10-20):(10-20):(40-70):(0-4). Like reaction zone B,reaction zone C may be realized not only in fixed bed reactors, but alsoin fluidized bed reactors.

The methacrolein conversion in reaction zone C based on single reactorpass (as always in this document) is customarily from 60 to 90 mol %.

It is quite generally the case that when gases recycled into thereaction zone A comprise O₂, this oxygen may be used in the reactionzone to selectively combust combustible substances such as hydrocarbon,coke, CO or preferably H₂ in reaction zone A, in order to thus generatethe heat of dehydrogenation required in reaction zone A. Advantageously,the methacrolein oxidation will be carried out with an appropriateoxygen excess so that the abovementioned residual gas recycled intoreaction zone A has a sufficient amount of oxygen for this purpose.

Frequently, the process according to the invention will be carried outin such a manner that at least 50 mol %, preferably at least 60 mol %,of the total amount of molecular oxygen introduced into the differentreaction zones has been converted in product gas mixture C.

EXAMPLES

1. Preparation of a Dehydrogenation Reactor

A solution of 11.993 g of SnCl₂.2H₂O and 7.886 g of H₂PtCl₆.6H₂O in 600ml of ethanol are poured over 1000 g of a spalled ZrO₂.SiO₂ mixed oxide.

The mixed oxide has a ZrO₂/SiO₂ weight ratio of 95:5. The mixed oxide ismanufactured by Norton (USA).

The mixed oxide has the following specification:

Type AXZ 311070306, Lot No. 2000160042, sieve fraction from 1.6 to 2 mm,BET surface area: 86 m²/g, pore volume: 0.28 ml/g (mercury porosimetrymeasurement).

The supernatant ethanol is taken off on a Rotavapor by rotating in awater jet pump vacuum (20 mbar) at a waterbath temperature of 40° C.Drying is then effected at 100° C. for 15 h and then calcining at 560°C. over 3 h, both under stationary air. A solution of 7.71 g of CsNO₃,13.559 g of KNO₃ and 98.33 g of La(NO₃)₃.6H₂O in 2500 ml of H₂O is thenpoured over the dry solids. The supernatant water is taken off on aRotavapor by rotating in a water jet pump vacuum (20 mbar) at a watertemperature of 85° C. Drying is then effected at 100° C. for 15 h andthen calcining at 560° C. over 3 h, both under stationary air.

The resulting catalyst precursor has a composition ofPt_(0.3)Sn_(0.6)Cs_(0.5)K_(0.5)La_(3.0) (stoichiometric coefficientsrepresent weight ratios) on (ZrO₂)₉₅.(SiO₂)₅ (stoichiometriccoefficients represent weight ratios).

20 ml of the catalyst precursor obtained are used to charge a verticaltube reactor (reactor length: 800 mm; wall thickness: 2 mm, internaldiameter: 20 mm; reactor material: internally alonized (i.e. aluminumoxide-coated) steel tube; heating: electrical (furnace from HTM Reetz,LOBA 1100-28-650-2) to a longitudinal average length of 650 mm; lengthof the catalyst bed: 75 mm; position of the catalyst bed: at thelongitudinal midpoint of the tubular reactor; filling of the remainingreactor volume above and below with steatite spheres (inert material) of4-5 mm diameter, supported from below on a catalyst base).

The reaction tube is then charged at an external wall temperature alongthe heating zone of 500° C. under closed loop control (based on a tubeflowed through by an identical inert gas stream) with 9.3 l/h (STP) ofhydrogen over 30 min. The hydrogen is then replaced at constant walltemperature firstly by a 23.6 l/h (STP) stream of 80% by volume ofnitrogen and 20% by volume of air over 30 min and then by an identicalstream of pure air over 30 min. While maintaining the wall temperature,purging is then effected using an identical stream of N₂ over 15 min andfinally reduction using 9.3 l/h (STP) of hydrogen again over 30 min. Theactivation of the catalyst precursor is then complete. This results in adehydrogenation reactor charged with dehydrogenation catalyst A(reaction zone A reactor).

2. Preparation of a Reaction Zone B Reactor

a) Preparation of a Starting Composition B1

To prepare the starting catalyst B1, 2000 g of (NH₄)₆Mo₇O₂₄.4H₂O aredissolved in portions in 5.4 l of water at 60° C. and admixed withstirring with 9.2 g of a 47.5% by weight aqueous KOH solution at 20° C.and then with 387.8 g of a 47.5% by weight aqueous CsNO₃ solution at 20°C. while maintaining the temperature at 60° C. (starting solution 1). Astarting solution 2 is prepared by stirring 1123.6 g of an aqueous ironnitrate solution (13.8% by weight of Fe) into 2449.5 g of an aqueouscobalt nitrate solution (12.5% by weight of Co) at 60° C. whilemaintaining the temperature at 60° C.

Within a period of 30 min, the starting solution 2 at 60° C. is stirredinto the starting solution 1 at 60° C. 15 min after stirring has ended,157.0 g of silica sol (density: 1.39 g/ml; pH=8.8; alkali metal content:≦0.5% by weight, 50.0% by weight of SiO₂; manufacturer: Dupont;Ludox®TM) are stirred into the aqueous suspension obtained (at 60° C.).The aqueous mixture is then stirred for a further 15 minutes. Theaqueous suspension is then spray-dried (exit temperature: 110° C., spraydryer from Niro DK; model: Niro A/S Atomizer Mobile Minor, centrifugalatomizer from Niro, DK), to obtain a spray powder of particle size from20 μm to 25 μm having a glow loss (3 h at 600° C. under air) of about30% by weight. This spray powder forms the starting composition B1.

b) Preparation of a Starting Composition B2

1715.6 g of tungstic acid (72.94% by weight of W) are added in portionsto 6344.6 g of an aqueous bismuth natrate solution in nitric acid (freenitric acid: 4% by weight, density: 1.24 mg/l; 11.2% by weight ofbismuth) at 20° C. with stirring. This gives an aqueous suspension whichis stirred at 20° C. for a further 2 h. This is then dried by spraydrying (exit temperature: 110° C., manufacturer: Niro DK; model: NiroA/S Atomizer Mobile Minor, centrifugal atomizer from Niro, DK). In thismanner, a spray powder of particle size from 20 μm to 25 μm is obtainedwhich has a glow loss (3 h at 600° C. under air) of about 12% by weight.After addition of 37 g of water, 400 g of this powder are kneaded usinga Werner & Pfleiderer LUK 075 kneader (kneader has two sigma bladesoperating in contrarotation) for 30 min. After the kneading, the kneadedmaterial is roughly divided and dried for 2 h in a drying cabinet(Binder, DE, type: FD 53) at 120° C. The entire amount of the driedmaterial is calcined in a muffle furnace from Nabertherm, capacity about120 l, at 800° C. over 2 h under an air stream of 1000 l/h (STP). Theair stream is at about 20° C. when it is passed into the muffle furnace.Heating to the calcination temperature is effected linearly from 25° C.within 8 h.

The calcined material is then milled to a number average particle size(narrow distribution, longest dimension) of about 5 μm and mixed with 1%by weight (based on the SiO₂-free composition) of finely divided SiO₂(bulk density: 150 g/l; number average particle size: 10 μm (longestdimension, narrow distribution); BET surface area: 100 m²/g).

This mixture forms the starting composition B2

c) Catalyst Preparation

1096 g of starting composition B1 and 200 g of starting composition B2are mixed homogeneously with the addition of (based on the overallcomposition of B1 and B2 used) 1.5% by weight of finely divided graphite(according to sieve analysis min. 50% by weight <24 μm; 24 μm <max. 10%by weight <48 μm; 5% by weight >48 μm; BET surface area: 10 m²/g) as atableting aid. This gives a mixture which has the following molarelemental stoichiometry (after calcination):[Bi₂W₂O₉.2 WO₃]_(0.5)[Mo₁₂Co_(5.5)Fe_(2.94)Si_(1.59)Cs₁K_(0.08)O_(x)]₁·C_(y).

Circular solid tablets of diameter 16 mm and height 3 mm are pressedfrom the mixture. The pressing pressure is 9 bar. The tablets arecomminuted and sieved through a sieve (0.8 mm mesh size). The materialwhich passes through the sieve, after addition of 2% by weight ofgraphite (based on the weight of the material which passes through thesieve) is tableted in a tablet pressing machine (Kilian S100, pressingforce: 15-20 N) into cylindrical rings of geometry 5 mm (externaldiameter)×3 mm (height)×2 mm (hole diameter).

150 g of these rings are calcined in a forced-air oven (Nabertherm,about 80 1 capacity) as follows:

-   a) linear heating is effected from room temperature to 180° C.    within 2 h and this temperature is maintained for 1 h;-   b) linear heating is then effected from 180° C. to 210° C. within 1    h and this temperature is maintained for 1 h;-   c) linear heating is then effected from 210° C. to 250° C. within 1    h and this temperature is maintained for 1 h;-   d) linear heating is then effected from 250° C. to 450° C. within    1.5 h and this temperature is maintained for 10 h;-   e) finally, the oven is left to cool by itself to room temperature    (about 25° C.).

During the entire calcination, 150 l/h (STP) of air are passed throughthe oven.

The end product forms the multimetal oxide catalyst B to be used inreaction zone B.

d) Charging of the Reaction Zone B Reactor

A vertical reactor tube (tube length: 1500 mm; wall thickness: 2.5 mm;internal diameter: 15 mm; reactor material: V2A steel; in a furnace fromHTM Reetz at a longitudinal midpoint length of 1300 mm, the remainingtube length at the tube entrance and the remaining tube length at thetube exit are heated with electrical heating bands) is charged with 100g of catalyst B. The length of the catalyst bed is 650 mm. The positionof the catalyst bed in the reaction tube is at the longitudinalmidpoint. The remaining reaction tube volume above and below is filledwith steatite spheres (inert material; 2-3 mm diameter), and the entirereaction tube charge is supported from below on a catalyst base of 10 cmheight.

3. Preparation of a Reaction Zone C Reactor

a) Preparation of a Starting Composition C1

4620 g of (NH₄)₆Mo₇O₂₄·4H₂O and 153.2 g of NH₄VO₃ are dissolved at 60°C. with constant stirring in 5 l of water preheated to 60° C. Whilemaintaining the temperature at 60° C., 421.7 g of a 76% by weightaqueous H₃PO₄ solution are added dropwise to this solution within 1 minwith stirring. First 10.9 g of diammonium sulfate and then 317.8 g ofpulverulent Sb₂O₃ (senarmontite) are then incorporated. The resultingmixture is then heated to 90° C. within 30 min (mixture 1). In parallel,424.9 g of CsNO₃ are dissolved at 90° C. in 850 ml of water preheated to90° C. to give a solution 1. While maintaining the temperature at 90°C., 446.5 g of an aqueous copper nitrate solution (15.5% by weight ofcopper) at 90° C. are added dropwise to mixture 1 within 4 min withcontinuous stirring. Spray-drying is then effected at an exittemperature of 110° C. (Niro DK, model: Niro A/S Atomizer Mobile Minor,centrifugal atomizer from Niro, DK). A spray powder of particle sizefrom 20 to 30 μm is obtained which forms the starting composition C1 andhas the following molar elemental stoichiometry (after calcination):Mo₁₂P_(1.5)V_(0.6)CS₁Cu_(0.5)Sb₁S_(0.04)O_(x).b) Preparation of a Starting Composition C2

880 g of pulverulent Sb₂O₃ (senarmontite) having an Sb content of 83% byweight are suspended with stirring in 4 l of water at 20° C. Whilemaintaining the temperature at 20° C., a solution of 670.5 g ofCu(NO₃)₂·2H₂O in 4 l of water is stirred into the suspension. This givesan aqueous suspension which is stirred at 80° C. for a further 2 h andis then spray-dried (exit temperature: 110° C., Niro DK, model: Niro A/SAtomizer Mobile Minor, centrifugal atomizer from Niro, DK). The spraypowder has a particle size of 20-30 μm. 700 g of the spray powder arethermally treated in a cylindrical rotary furnace (length of thecalcination chamber 0.50 m; internal diameter: 12.5 cm) while passingthrough 200 l/h (STP) of air as follows: within 1 h, linear heating iseffected to 150° C. Linear heating is then effected to 200° C. within 4h. Afterwards, linear heating is effected to 300° C. within 2 h and thento 400° C. within 2 h. Finally, linear heating is effected to 900° C.within 48 h.

After cooling to room temperature, a powder is obtained which has aspecific BET surface area of 0.3 m²/g. This powder forms the startingcomposition C2 and substantially has the diffraction reflections ofCuSb₂O₆ (comparative spectrum 17-0284 from the JCPDS-ICDD index). Thestarting composition C2 has the following molar elemental stoichiometry:CuSb₂O₆.c) Catalyst Preparation

The amounts of the starting composition C1 and the starting compositionC2 corresponding to the mixing stoichiometry (after calcination)(Mo₁₂P_(1.5)V_(0.6)Cs₁Cu_(0.5)Sb₁S_(0.04)O_(x))₁·(Cu₁Sb₂O₆)_(0.5) areintimately mixed. 2% by weight of finely divided graphite (according tosieve analysis, min. 50% by weight <24 μm; 24 μm <max. 10% by weight <48μm; 5% by weight >48 μm; BET surface: 10 m²/g) are then admixed into 500g of the abovementioned mixture as a tableting aid.

A tablet pressing machine (Kilian S 100) is used without the addition offurther additives to shape cylindrical rings of geometry 7 mm (externaldiameter)×7 mm (height)×3 mm (hole diameter) from the mixture.

500 g of the rings are then thermally treated in a forced-air oven(Nabertherm, capacity about 80 l) under a constant air stream (500 l/h(STP)·kg of solid) as follows: heating is effected at 4° C./min from 25°C. to 270° C. while maintaining the intermediate temperatures of 180° C.and 220° C. and the end temperature of 270° C. for 30 minutes each.Finally, the temperature is increased at a rate of 2° C./min to 370° C.and this temperature is maintained over 6 h.

Cooling is then effected to room temperature and the hollow cylindersare processed to spall having a longest dimension of 1.6-3 mm. Thisspall forms a multimetal catalyst C to be used in reaction zone C.

d) Charging of the Reaction Zone C Reactor

A vertical reactor tube (tube length: 1800 mm; wall thickness: 1 mm;internal diameter: 8 mm; reactor material: V2A steel; in a furnace fromHTM Reetz at a longitudinal midpoint length of 1600 mm) is charged with75 g of the multimetal oxide catalyst C. The length of the catalyst bedis 1000 mm. The position of the catalyst bed in the reaction tube is atthe longitudinal midpoint. Above and below, the remaining reaction tubevolume is filled with steatite spheres (inert material; diameter: 2-3mm), and the entire reaction tube charge is supported from below on acatalyst base of height 10 cm. The remaining tube length at the tubeentrance and the remaining tube length at the tube exit is heated withelectrical heating bands.

4. Performance of the Process According to the Invention

A) The reaction zone A reactor from 1. at an external wall temperaturealong the heating zone of 500° C under closed loop control (based on atube flowed through by an identical inert gas stream) is charged with areaction gas mixture of 20 l/h (STP) of isobutane, 10 l/h (STP) of airand 8 g/h of steam.

The isobutane is metered in using a mass flow regulator from Brooks,while the water is first metered into an evaporator using an HPLC pump420 from Kontron, evaporated in it and then mixed with the isobutane andthe air. During the charging, the temperature of the charging gasmixture is 150° C. The starting pressure in the tube reactor is set to1.5 bar by means of a pressure regulator from REKO disposed at thereactor exit.

Downstream of the pressure regulator, the product gas mixture A isdepressurized to atmospheric pressure and cooled, and the steamcontained therein condenses out. The gas remaining is analyzed by meansof GC (HP 6890 with Chem.-Station, detectors: FID; TCD, separatingcolumns: Al₂O₃/KCl (Chrompack), Carboxen 1010 (Supelco)). In acorresponding manner, the charging gas mixture is also analyzed.

After an operating time of three weeks, the following analytical resultsare obtained:

Charging gas mixture Product gas mixture A (% by volume) (% by volume)isobutane 50 33 isobutene — 10 Nitrogen 20 17.5 Steam 25 26 Oxygen 5 —CO — <0.1 CO₂ — 2.5 H₂ — 11 Propene — <0.1 Propane — <0.1 Ethene — <0.1Ethane — <0.1These values correspond to an isobutane conversion based on a singlepass of 25 mol % and a selectivity of isobutene formation of 90 mol %.

The portion of the reaction zone B reactor from 2. in the oven ismaintained by closed loop control at an external wall temperature (basedon a tube flowed through by an identical inert gas stream) at which theisobutene conversion at a single pass of the reaction mixture is 98 mol%. The heating band at the reaction tube entrance (where the catalystbase is disposed) is likewise set to this temperature and the heatingband at the reaction tube exit is set to a temperature 50° C. lower. Allcomponents other than isobutane and isobutene are removed from theproduct gas mixture A.

The remaining mixture of 15 1/h (STP) of isobutane and 4.5 l/h (STP) ofisobutene together with 45 l/h (STP) of air and 7 l/h (STP) of steamform the charging gas mixture for the reaction zone B reactor. Thetemperature of the charging gas mixture is increased to the reactorexternal wall temperature. The starting pressure in the reactor is setto 1.3 bar by means of a pressure regulator disposed at the reactorexit.

Downstream of the pressure regulator, the product gas mixture B(temperature =300° C.) is depressurized and analyzed by means of GC (HP6890 with Chem.-Station, detectors: TCD, FID, separating columns:Poraplot Q (Chrompack), Carboxen 1010 (Supelco)). In an identicalmanner, the charging gas mixture is also analyzed.

After an operating time of 3 weeks, the following results are obtained:

Charging gas mixture Product gas mixture B (% by volume) (Vol.-%)isobutane 21 20.5 isobutene 6.5 <0.1 H₂ — — O₂ 12.5 3 N₂ 50 49 H₂O 10 19Methacrolein — 5.2 Methacrylic acid — <0.1These values correspond to an isobutene conversion based on single passof 98 mol % and a selectivity of methacrolein formation of 84 mol %.

The portion of the reaction C reactor in the furnace is maintained byclosed loop control at an external wall temperature of 290° C. (based ona tube flowed through by an identical inert gas stream).

The heating band at the reaction tube entrance (where the catalyst baseis disposed) is set to 290° C. and the heating band at the reaction tubeexit is set to 200° C.

The charging gas mixture of the reaction zone C reactor consists of 29l/h (STP) of air and the product gas mixture B (73 l/h (STP)). Thetemperature of the charging gas is increased to 290° C.

The pressure at the reaction tube exit is set to 1.3 bar using apressure regulator disposed at the reactor exit.

Downstream of the pressure regulator, the product gas mixture C(temperature: 200° C.) is depressurized and analyzed by means of GC (HP6890 with Chem.-Station, detectors: TCD, FID, separating columns:Poraplot Q (Chrompack), Carboxen 1010 (Supelco)). In a correspondingmanner, the charging gas mixture is also analyzed.

After an operating time of 3 weeks, the following results are obtained:

Charging gas mixture Product gas mixture C (% by volume) (% by volume)N₂ 58 58 O₂ 8 6 H₂O 14 14 Methacrolein 3.5 1.4 Methacrylic acid <0.1 1.7These values correspond to a methacrolein conversion based on singlepass of 60 mol % and a selectivity of methacrylic acid formation of 81mol %.

5. Comparative Examples

A) The process according to the invention according to 4. is repeated.However, when the reaction zone B reactor is charged, the charging gasmixture used is a mixture of the entire product gas mixture A (41 l/h(STP) of which a proportion of 11 l/h (STP) is steam) and 45 l/h (STP)of air.

The loading of the catalyst charge in the reaction zone B reactor withisobutene and oxygen is accordingly 4.5 l/h (STP) of isobutene and 9 l/h(STP) of oxygen as in 4. and the loading of the catalyst charge in thereaction zone C reactor with methacrolein and oxygen is 3.8 l/h (STP) ofmethacrolein and 8 l/h (STP) of oxygen as in 4.

After an operating time of 3 weeks, the following results are obtained:

Charging gas mixture Product gas mixture C (% by volume) (% by volume)N₂ 55 55 O₂ 6.5 4.5 H₂O 15 15.5 Methacrolein 3 1.8 Methacrylic acid <0.10.9These values correspond to a methacrolein conversion based on singlepass of 40 mol % and a selectivity of methacrylic acid formation of 75mol %.

B) The process according to the invention according to 4. is repeated,except that instead of 45 l/h (STP) of air, 9 l/h (STP) of puremolecular oxygen are used for the charging gas for the reaction zone Breactor. The total isobutyraldehyde+isobutyric acid content of theproduct gas mixture C is perceptibly increased compared to theexperimental procedure in 4.

1. A process for preparing methacrylic acid from isobutane comprising A)subjecting isobutane in a reaction zone A to a partial heterogeneouslycatalyzed dehydrogenation in the gas phase to form a product mixture Awhich comprises isobutene and unconverted isobutane, B) charging areaction zone B with the product gas mixture A and subjecting theisobutene in reaction zone B to a heterogeneously catalyzed partialoxidation in the gas phase with molecular oxygen to form a product gasmixture B comprising methacrolein, wherein the molar conversion ofisobutene is ≧95 mol% and C) charging a reaction zone C with the productgas mixture B without prior removal of components contained therein andsubjecting the methacrolein in a reaction zone C to a heterogeneouslycatalyzed partial oxidation with molecular oxygen in the gas phase toform a product gas mixture C comprising methacrylic acid, wherein atleast 80 mol% of the components other than isobutane and isobutene inthe product gas mixture A are removed before the charging to reactionzone B, and wherein the molecular oxygen is introduced to reaction zoneB as a mixture with molecular nitrogen in a molar ratio Rof molecularoxygen to molecular nitrogen of from 1:1 to 1:10.
 2. The process asclaimed in claim 1, wherein R is from 1:3 to 1:10.
 3. The process asclaimed in claim 1, wherein R is from 1:3 to 1:6.
 4. The process asclaimed in claim 1, wherein in zone B the molecular oxygen comprisesair.
 5. The process as claimed in claim 1, wherein at least 90 mol% ofthe components other than isobutane and isobutene present in the productgas mixture A are removed before the charging to reaction zone B.
 6. Theprocess as claimed in claim 1, wherein at least 95 mol% of thecomponents other than isobutane and isobutene present in the product gasmixture A are removed before the charging to reaction zone B.
 7. Theprocess as claimed in claim 1, wherein in reaction zone A from ≧5 mol%to ≦30 mol% of isobutane is converted to isobutene.
 8. The process asclaimed in claim 1, further comprising contacting the product gasmixture A with an organic solvent to absorb isobutane and isobutene, andfreeing the isobutane and isobutene from the organic solvent bysubsequently desorbing, stripping or both, and charging the freedisobutene and isobutane to reaction zone B.
 9. The process as claimed inclaim 1, wherein at least 97 mol% of the isobutene is oxidized in thereaction zone B.
 10. The process as claimed in claim 1, wherein at least98 mol% of the isobutene is oxidized in reaction zone B.
 11. The processas claimed in claim 1, further comprising supplementing the product gasmixture B with air before charging to reaction zone C.